Eddy currents provide a measurable parameter indicative of flaws in the surface and sub-surface of materials. Eddy currents are generally confined to the surface and near surface regions of the material. Eddy currents are affected by changes in the resistivity of the material. Flaws in the material, such as microscopic hair line cracks or pits, affect the localized resistivity of the material. Flaws in a material cause localized variations in the eddy currents in the material. Accordingly, a material can be inspected for flaws by inducing and measuring eddy currents in the material.
Eddy current probes detect material flaws by sensing variations in eddy currents. These probes have coils with high frequency currents that project a fluctuating magnetic field into the conductive material being measured. This imposed magnetic field induces eddy currents in the material. The strength of the eddy currents depends on the local resistivity of the material which is affected by the presence of material flaws. These eddy currents create a magnetic field that varies in intensity with the strength of the eddy current and, hence, the presence of material flaws.
The magnetic field created by the eddy currents extends above the material surface to the probe. Thus, the magnetic field from the eddy current induces its own voltage in the probe coil. The eddy magnetic field opposes the coil field. These coupled magnetic fields measurably affect the net current and inductance of the probe coils. These variations in the coil currents vary in response to material flaws and, thus, are measured to detect these flaws.
For the probe coil current to reliably indicate variations in eddy currents, other parameters that affect the coil current must be constant. One such parameter is the distance between the face of the probe and the surface of the material. The degree of coupling between the magnetic fields from the coil current and the eddy current depends on the gap between the probe and the material with the eddy currents. The gap between the probe and material is known in the art as being the "lift-off" of the probe. Prior probes do not have a reliably constant lift-off. Lift-off has long-posed difficulties to eddy current probes that have not been solved until now.
Changes in the lift-off gap alter the amount of magnetic coupling and, hence, the coil current. The eddy current probe detects current alterations due to gap changes just as it detects variations in the eddy current due to material flaws. Since it is desired to detect only eddy current alterations due to material flaws, variations due to changes in the lift-off gap must be prevented. Accordingly, the gap between the probe and the material surface must be held constant to ensure proper operation of the probe.
It is difficult to maintain a constant distance between the probe and the material being-tested. It is particularly difficult to maintain a constant gap when a large surface, such as a retaining ring for a power generator, is being tested. These retaining rings are large and typically have radii in the range of 13 to 36 inches. Because they are so large and their surfaces may have been reworked from earlier service, retaining rings are not perfect cylinders. The surface of a retaining ring is immense compared to the small material flaws that an eddy current probe is to detect. There are small irregularities in the shape of the retaining ring that deform the ring from a true cylinder. Similarly, large retaining rings do not have surfaces that are uniform at the small order of magnitude (microcracks) at which the eddy currents are being measured. The irregularities in the shape and surface of the retaining ring have made it difficult to hold the probe at a constant distance from the surface of the ring.
Eddy current probes are usually fixed with respect to a reference other than the retaining ring. A true and known reference is necessary to precisely position the probe with respect to the retaining ring. The retaining ring usually bears a stamp on its end surface marking the zero degree position of the ring. The position of the eddy current probe is referenced from this zero reference stamp. A fixed reference for the probe is established with a conventional reference frame. This reference frame is attached to the retaining ring and is centered on the axis of the ring as is shown in FIG. 1. The eddy current probe is affixed to the reference frame and positioned near the surface of the retaining ring. The reference frame is motorized so that the eddy current probe can be drawn across the surface of the retaining ring. Generally, the probe is moved axially along the length of the retaining ring in a straight scan line.
As the probe completely traverses each scan line across the retaining ring surface, the probe is radially indexed to the next scan line around the reference frame. The probe is then drawn along this next scan line. This scanning and indexing sequence is repeated until the probe has completed scans around the entire circumference of the retaining ring. In this way, the probe covers the entire surface of the retaining ring. The probe must cover the entire ring to ensure that all material flaws are detected. To do this, the probe must travel along straight scan lines. If the probe wanders off a scan line, then portions of the material surface will be missed by the probe and flaws in the material may escape detection.
Prior to the present invention, probes have either hovered over the surface of the ring or been biased against the surface of the ring. Probes that hovered over the ring surface were held solely by the reference frame and were not directly in contact with the retaining ring. It is exceedingly difficult to maintain a constant lift-off gap between a hovering probe and the ring surface because of the irregularities in the shape and surface of the retaining ring. To maintain a constant gap between the probe and ring, other probes are pressed against the surface of the ring by the reference frame. The gap between the probe and ring surface is held constant because the probe slides directly on the surface. However, these probes rub against the surface, wear out quickly and collect dirt in the face of the probe. This rubbing also creates vibration in the probe which affects the coil current being measured. The mechanical vibration from the probe creates signal noise within the measured coil current. This noise tends to obscure the desired eddy current signals. Accordingly, until the present invention, a real need existed for an eddy current probe that did not rub against a surface, was held a constant distance from the surface of the ring and was positioned in a fixed reference frame.
In the present invention, a carriage carries the eddy current probe a fixed distance above the retaining ring surface. The carriage has self-lubricating feet that slide on the ring surface. The feet are close enough together so that large scale irregularities in the shape of the ring do not vary the gap between probe and retaining ring surface. The feet of the carriage are far enough apart so that small surface defects do not jolt or otherwise disturb the probe. In addition, this carriage can be slid across a material surface at a much-faster speed than can conventional eddy current probes.
The carriage is moved in a straight line along the surface of the retaining ring. A conventional motorized reference frame having shafts parallel to the ring axis and to the path of the carriage carries the carriage across the surface of the ring. The carriage is coupled to these shafts by a rod extending perpendicularly from the carriage. The rod is attached to the reference frame and the carriage. There are limited degrees of freedom of movement between the carriage and the rod to allow the carriage to ride on an irregular surface. However, the carriage is not permitted to move in any direction that would divert the eddy current probe from its intended straight scan line. Accordingly, as the carriage moves across the surface of the retaining ring, the eddy current probe is held to a predetermined straight scan line. This straight line movement allows the entire surface of a material be traversed without missing portions of the surface due to a wandering probe.
Some signal noise will always be present in the coil current. It is not practical to mechanically eliminate all of the sources of signal noise. Accordingly, signal processing techniques are used in the present invention to discriminate the current signals attributable to variations in the eddy current from noise and other undesirable signals. The principal signal processing technique employed in the present invention is to compare two coil signals that are nearly identical but for the desired eddy current signal. A new split coil eddy current probe provides these two similar current signals. This new probe is superior to prior differential probes such as is described in U.S. Pat. No. 4,855,677.
The present split coil probe has two separate but identical coils positioned side-by-side on the face of the probe. Unlike conventional cylindrical differential coils, the present coils have "D" shapes that allow them to be more closely packed together within the probe. This close packing of coils improves the spatial resolution of the probe because its electrical radius is smaller than that of conventional cylindrical differential coils.
Both coils have the same drive current and are drawn along adjacent parallel paths over the surface of the retaining ring. Both coils are magnetically coupled to the eddy currents they each separately induce in the retaining ring. The gap between the ring surface and the probe is the same for both coils. Accordingly, the coil currents for each coil is substantially the same.
The two coils are far enough apart so that they will not pass over the same material flaws in the ring surface at the same time. Although the coils are side by side and very close together, they do not project overlapping magnetic fields onto the same portion of the ring surface. The magnetic fields generated by the each of coils and projected against the ring surface has a shape substantially the same as the "D" shaped end of the coil. The side-by-side coils project side by side magnetic fields onto the ring surface. The opposing "D" shapes of the coils allows the projected magnetic fields to abut against one another without overlapping.
Since the flaws in the retaining ring material tend to be microscopic, individual flaws generally do not traverse across the side-by-side magnetic fields. When one probe coil passes over a material flaw, the other coil does not pass over the same flaw. Since material flaws affect the eddy currents that magnetically couple a coil, the current in the coil passing over the flaw is affected by the altered eddy current while the other coil current is not affected by the flaw. Accordingly, the difference between the two coil current signals is due to microflaws in the ring surface and sub-surface.
In addition, most of the noise and other signal effects in the eddy current probe can be masked from the coil signal by using impedance bridge and amplifier circuits to process the coil current signals. These circuits are contained in a conventional eddy current instrument such as an MIZ-40 model instrument manufactured by the Zetec Corp. of Issaquah, Wash. To further refine the signals from the eddy current probe, the coil signals from the bridge circuit are passed through two synchronous differential amplifier circuits to create two difference signals. One amplifier is synchronized with the drive oscillator. The in-phase signal, after the addition of a user selectable display phase angle (.phi.), can be rotated on the display screen so that it generally corresponds to liftoff variations between the two coils (to the extent that such variations exist with side-by-side coils) and other noise.
The second synchronous differential amplifier has a 90.degree. phase shift with the drive oscillator and, thus, compares the out-of-phase differences between the coil signals. The out-of-phase signal, after the addition of the same user selectable display phase angle (.phi.), can be rotated on the display so that it generally corresponds to variations in the eddy currents between the two coils and which are due to material flaws. Since the eddy currents are generated by and magnetically coupled to the coil current, the eddy currents are slightly behind the phase of the coil current. The eddy currents tend to retard the coil current because of the magnetic coupling. Thus, the currents in the two coils will be out-of-phase due to the eddy currents.
The processed signal data from the eddy current probe is displayed via conventional display means. Strip charts have been used to show each scan line of the probe and show where the eddy current varies with respect to the material surface. Similarly, CRT display screens can be used to present the eddy current data. The display may be adjusted to show the in-phase and out-of-phase differential signals on respective horizontal and vertical display axes to enhance the user's ability to analyze the data. In addition, a computer can be used to display the signals, and to color-code and plot the signals for a display or to print a paper copy of the data. The displays have in common the presentation of data indicative of material flaws in the ring. The data may be presented such that the location of the flaws in the material is apparent or may be presented such that the area and extent of the flaws are apparent.
It is an object of the present invention to provide an improved eddy current probe, carriage and signal processor. The probe has split coils that enable the signal processor to reliably identify the eddy current variations indicative of defects in the material being scanned. The carriage maintains a constant lift-off gap between the probe and the surface of the material. The carriage also keeps the probe on a straight scan line while adjusting to variations in the surface of the material.